The present invention relates to a fuel injection system for a spark plug-ignited internal combustion engine that operates on fuel continuously injected into the suction pipe (air intake pipe), in which an air sensor and an arbitrarily operable butterfly valve are disposed in series. The air sensor is displaced against a return force in proportion to the quantity of air flowing through the suction pipe. In the course of its excursion, the air sensor displaces a control plunger part of a fuel metering valve disposed in the fuel supply line, which meters a fuel quantity which is proportionate to the quantity of throughgoing air. The afore-noted return force is provided by a liquid which is continuously delivered under constant but arbitrarily variable pressure through a control pressure line and which exerts the return force on the control plunger. The pressure of the liquid is variable by at least one pressure control valve controllable as a function of the engine characteristics. The pressure control valve is in the form of a flat seat valve having a diaphragm as its movable valve part.
This type of fuel injection system is designed to automatically provide a good fuel-air mixture for all the operating conditions of the internal combustion engine so as to obtain complete combustion of the fuel and thus, with the least possible fuel consumption, to prevent the production of toxic exhaust gases, or at least to considerably reduce the same. The quantity of fuel must therefore be very accurately metered in accordance with the requirements of each operating state of the internal combustion engine and the air to fuel ratio must be varied as a function of operating parameters such as speed, load, temperature and exhaust gas composition.
In the case of known fuel injection systems of this type, the quantity of fuel which is metered out is, as far as possible, proportionate to the quantity of air flowing through the suction pipe. The ratio of the quantity of fuel metered to the quantity of air may be varied by changing the return force of the metering element as a function of the operaing parameters of the internal combustion engine by means of an electromagnetic pressure control valve.